Composite materials commonly consist of a continuous, bulk or matrix phase, and a discontinuous, dispersed, fiber, or reinforcement phase, and can be produced by mixing two immiscible polymers. Some composites have a relatively brittle matrix and a relatively ductile or pliable reinforcement. The relatively pliable reinforcement, which can be in the form of fibers, can serve to impart toughness to the composite. Specifically, the reinforcement may inhibit crack propagation as cracks through the brittle matrix are deflected and directed along the length of the fibers. Other composites have a relatively soft matrix and a relatively rigid or strong reinforcement phase, which can include fibers. Such fibers can impart strength to the matrix, by transferring applied loads from the weak matrix to the stronger fibers.
Microfibers that impart additional strength may be formed of polymers, metals, or other materials. Many materials, such as metals, have the disadvantage of relatively high weight and density. Other materials, such as glass, may be inexpensive and lighter, but may wick moisture into the composite, which may then make the composite unsuitable for some applications. In particular, long-term submersion in water may lead to significant water uptake and decomposition, including delamination in some applications. The wicking may be caused by less than optimal adhesion between the fibers and the matrix phase, allowing moisture to be wicked in through the elongated voids formed between the fibers and the matrix. Use of inexpensive polymers, such as polyolefins or polyesters would be advantageous with respect to cost and weight, but known olefin fibers that are strong enough to impart the required strength to the composite may not be capable of receiving stress from the matrix, because of the low surface energy nature of known olefin fiber surfaces. Inexpensive polymer fibers such as polyolefin or polyester fibers may also allow wicking of moisture even though they are hydrophobic in nature.
The production of polymer microfibers from polymer films is well known. Typically, molten polymer is extruded through a die or small orifice in a continuous manner to form a continuous thread. The fiber can be further drawn to create an oriented filament with significant tensile strength. Fibers created by a traditional melt spinning process are generally larger than 15 microns. Smaller fiber sizes are impractical because of the high melt viscosity of the molten polymer. Fibers with a diameter less than 15 microns can be created by a melt blowing process. However, the resins used in this process have a low molecular weight and viscosity rendering the resulting fibers very weak. In addition, a post spinning process such as length orientation cannot be used.
One typical method to obtain in situ composites is to blend thermotropic liquid crystal polymers (TLCPs) with thermoplastic polymer (TP) matrixes [see, e.g., G. Kiss, Polymer Engineering and Science, 27:410 (1987); Y. Qin et al., Polymer, 34:1196-1201 (1993); Y. Qin et al., Polymer, 34:1202-1206 (1993); Markku T. Heino et al., Journal of applied polymer science, 51:259-270 (1994); F. J. Vallejo et al., Polymer, 41:6311-6321 (2000)]. Considering the high cost of TLCPs for industry applications, replacing the TLCPs with general engineering TP to prepare in situ microfibrillar composites is highly desirable.
Another method to prepare in situ microfibril reinforced composites is through a melt extrusion-cold drawing-thermal treatment process [see, e.g., M. Evstatiev and S. Fakirov, Polymer, 33:877-880 (1992); K. Friedrich et al., Composites Science and Technology, 65:107-116 (2005); S. Fakirov and M. Evstatiev, Polymer, 34:4669-4679 (1993); S. Fakirov et al., Macromolecules, 26:5219-5226 (1993); M. Evstatiev and N. Nicolov, Polymer, 37:4455-4463 (1996); S. Fakirov et al., Journal of Macromolecular Science, Part B-Physics, B43:775-789 (2004); M. Evstatiev et al., Advances in Polymer Technology, 19:249-259 (2000); A. A. Apostolov et al., Progr Colloid Polym Sci, 130:159-166 (2005); M. Krumova et al., Progr Colloid Polym Sci, 130:167-173 (2005); K. Friedrich et al., Composites Science and Technology, 65:107-116 (2005); Z. M. Li et al., Materials Research Bulletin, 37:2185-2197 (2002)].
Pennings et al., in “Mechanical properties and hydrolyzability of Poly(L-lactide) Fibers Produced by a Dry-Spinning Method” J. Appl. Polym. Sci., 29, 2829-2842 (1984) described fibers with a fibrillar structure by solution spinning using chloroform in the presence of various additives (camphor, polyurethanes) followed by hot drawing. These fibers showed good mechanical properties and improved degradability in vitro with the fibrillar structure speeding up the hydrolysis of the fiber. The inherent disadvantage of this process is the use of chlorinated solvents in the spinning process. Composite fiber with in situ microfibril provide a promising method to prepare microfibers.
Microfibers with a diameter of 1 micrometer and a round cross-section have also been produced by electrospining. The electrospining technique suffers from the disadvantage of using a chlorinated solvent and has low production speed. In view of the foregoing, there is a need to develop other efficient methods for production of microfibrillar composites, microfibers and nanofibers of thermoplastic polymers for applications in biosensors, membranes, filters, protein support, and organ repairs as well as for the manufacture of woven fabrics including biocidal textiles. The present invention satisfies these and other needs.